Field of the Invention
The present invention relates generally to valves for vacuum filters and, more specifically, to valves for rotating disc filters used in the production of pulp from a pulp slurry. Still more specifically, the present invention relates to an adjustable valve for a rotating disc filter that can selectively restrict the flow of filtrate from the pulp slurry through the disc filter segments.
In the pulp and paper industry, disc and other rotary type filters are used for two primary purposes: to thicken pulps by removing liquid under a combination of gravity and vacuum induced drainage; and to act as save-ails for recovering separated liquid into a number of filtrate streams containing varying amounts of particulate matter. Typical disc filters include two phases; an atmospheric draining where gravity is employed to drain filtrate from the pulp as it collects on the rotating disc; and a vacuum cycle where a vacuum is generated by a difference in the hydrostatic heads between the vat in which the rotating disc filter is contained and the barometric leg which is the conduit that drains filtrate collected in the central core of the disc filter.
Different pulps require different conditions to form a pulp mat on the disc segments and to optimize the clarity of the clear portion of the collected filtrate. Specifically, for optimal disc filter operation, a slow draining pulp requires a longer vacuum cycle and a fast draining pulp requires a shorter vacuum cycle. The duration of the vacuum cycle is determined by the timing of the closing of a valve disposed between the rotating disc segments and the core of the disc filter. Even with prior testing, it has become impossible to predict and design exactly the optimum vacuum cut off point for any given pulp. In addition, the operator of the plant may be forced to vary the pulp characteristics on a regular basis depending upon the product being manufactured and the raw materials being supplied. Thus, the traditional fixed vacuum cut off point or fixed valve timing in the rotational cycle of the disc filter is problematic because the incorrect timing of the vacuum cut off point will adversely effect pulp formation and filtrate clarity.
The complexity of this problem is illustrated in FIGS. 1-3. Referring to FIG. 1, a vat 10 is illustrated which is partially filled with a pulp slurry 11. A disc filter core 12 is further contained within a vacuum box 13, both of which are partially submerged in the slurry 11. The filter 12 includes a plurality of sectors 14 which are rotated in the direction of the arrow 15 through the pulp slurry 11. The sectors 14 are in communication with a drainage structure which features a centralized core 16. The core is in communication with a first barometric or drop leg 17, also known as the "cloudy" leg. The filtrate carried away by leg 17 has a higher concentration of fibers than that carried away by the second barometric leg 18, also known as the "clear" leg. The second leg 18 carries away filtrate that comes through the sheet formed on the sector 14 during the vacuum cycle as discussed below. The filtrate carried away by the second leg 18 is clearer than the filtrate carried away by the first leg 17 because of the increased filtering effect of the sheet or mat formed on the sector 14.
The sector 14 is formed of either a plastic mesh or stainless steel woven screen. The centralized core 16 is typically separated into segments 19. As the disc 12 is rotated, the sector enters the slurry and filtrate begins to fill the sector. This initial filtrate which includes some suspended pulp fiber, is pushed through the filter medium by the hydrostatic pressure head of the slurry 11 in the vat 10. At the very beginning of this stage, flow from the filter sector 14 to the core 16 is initially cut off by a valve 27 until a sufficient amount of filtrate is accumulated in the segment 19 and the sector 14. Then, as the disc 12 and core 16 rotate, a valve opening is passed and air accumulated in the disc segment 19 and the sector 14 is vented to the atmosphere during the phase marked A in FIG. 1. As discussed below, it is important to remove the excess air as it will decrease the vacuum provided by the drop legs 17, 18. Also occurring at this initial stage is the beginning of the formation of a sheet on the face of the filter sector 14. It is the formation of the sheet, in combination with the filter mesh, that permits nearly complete removal of the fiber from the filtrate. As the disc continues to rotate in the clockwise direction (see arrow 15), excess filtrate is removed through a gravity drain 22. As shown in FIG. 2, the gravity drain is not operated on vacuum.
Returning to FIG. 1, as the core 16 and sector 14 continue to rotate, a valve shown schematically at 23 closes off filtrate flow from the core 16 and filtrate flow is terminated. The core 16 at this point is filled with filtrate and the closure by the valve 23 lasts for only one sector 14 in the rotation cycle. As the core 16 and sector 14 pass the valve 23, the core channel is then reopened and a vacuum is provided by the hydrostatic pressure drop existing between the vacuum box 13 and the leg 17 at the start of the phase marked B and the leg 18 towards the end of the vacuum phase. The vacuum or pressure drop allows filtrate to drain through the disc sectors 14 and the segments 19 to the legs 17, 18. The vacuum becomes the driving force that pulls the filtrate through the sheet or mat that has formed on the disc. During this formation, the sheet is thickened and densified thereby acquiring material integrity. As the core 16 and sectors 14 continue to rotate, the loss of fiber through the sheet becomes minute thereby producing the "clear" filtrate that is carried away by the second leg 18.
A splitter plate 24 is provided to split the flow between the first leg 17 and second leg 18 in the region marked E. Some systems require a third barometric leg (not shown). This leg would carry away the last portions of filtrate collected under the vacuum, or just prior to the arrival of the segment 19 at the vacuum cut off valve 25 which will be discussed in detail below.
Continuing with the clockwise rotation of the filter 12, as the filter sector 14 emerges from the slurry 11 at region F, the sheet formed on the face of the filter will begin to dry out. A vacuum is maintained on the sector 14. The passage of air through the sheet formed on the sector 14 is limited due to the density of the sheet. As the segment approaches the valve 25 (see the region labeled G), some air will filter through the sheet allowing additional filtrate trapped in the sheet and sector 14 to be drained and pulled into the core 16. This air passage to the core is illustrated at 26 in FIG. 2. As the segment 19 traverses the cut off valve 25, flow from the core 16 is terminated. Then, the sheet will be removed by knock-off showers (not shown) that peel the sheet off the facing of the sector 14 thereby allowing the sheet to drop out of the vat 10 through stock discharge boxes (not shown). As the sector 14 and core 16 continues to rotate, the valve 25 is passed and flow from the core 16 occurs by gravity only for approximately one leaf cycle (see the region labeled H). Flow is then cut off again by the valve shown at 27 where the sector 14 is cleaned with a high pressure water stream (not shown). The screen cleaning takes place in the region labeled I.
As noted above, different pulps require different conditions in order to optimize mat or sheet formation and in order to optimize the clarity of the filtrate collected through the second leg 18. As indicated in FIGS. 3A and 3B, the timing of the vacuum cut off provided by the valve 25 is crucial. Further changes in the consistency of the slurry 11 or in the type of pulp being filtered can greatly affect the timing and the efficiency of the disc filter 12. As shown in both FIGS. 3A and 3B, if the timing is not accurate, air can leak from the filter segments into the core 16 and barometric leg 18, thereby destroying the vacuum provided by the leg 18. When the vacuum is destroyed in the leg 18, trapped filtrate in the leaf will backwash and force the formed sheet away from the face of the sector 14. A portion of the sheet may be lost into the vat 10 thereby increasing the free passage for air to proceed into the core 16. The vacuum provided by the leg 18 can be eliminated which would require a shut-down of the operation.
Referring to FIG. 3A, if the vacuum cut off provided by the valve 25 is too late or "too high", the lack of filtrate in the segment 19 will permit air to leak into the core thereby reducing the vacuum provided by the leg 18. As shown in FIG. 3B, if the cut-off provided by the valve 25 is too low, excess filtrate will remain in the segments 19 which will result in backwash of the filtrate because flow has been stopped which, in turn, will eliminate the residual vacuum in the structure of the sector 14. This backwash can cause loss of part of the sheet and clear filtrate back into the vat.
Until now, the only advances in controlling the vacuum cut-off point provided by the prior art is the design of the cut-off valves 25 themselves. Referring to FIGS. 3A and 3B, the valve 25 includes an open end 28 which permits the flow of filtrate through the segments 19 to the vacuum box 13. A tapered middle section 29 gradually restricts this flow. Finally, a closed end 31 provides the vacuum cut-off. It is the timing of the vacuum cut-off provided by the closed end 31 of the valve 25 that is crucial for the reasons discussed above.
Accordingly, there is a need for an improved method and apparatus for repositioning the vacuum cut-off valve during operation of the filter 12 to reduce the amount of filtrate lost back into the vat from rewetting of the sheet and further to control the amount of air passing into the core 16.